U.S. Pat. No. 4,764,394 to Conrad entitled "Method and Apparatus for Plasma Source Ion Implantation" discloses an ion implantation system for treating a target by means of ionic bombardment. Ion implantation into surfaces of a three dimensional target is achieved by forming an ionized plasma about the target within an enclosing chamber. Once the plasma is set up in a region surrounding the target, ions from the plasma are driven into the target object from all sides without need to manipulate the target. This implantation is accomplished by application of repetitive pulses of high voltage, typically ten kilovolts or less, that cause the ions to be driven into the exposed surfaces of the target. A technique discussed in the '394 patent for setting up the plasma is to introduce a neutral gas into the region of the target and then ionize the gas with ionizing radiation.
The system disclosed in the '394 patent to Conrad uses an ion source to create an ion plasma in a region surrounding a workpiece and then selectively pulses an electrode that supports the workpiece thereby attracting the positive ions in the plasma toward the workpiece.
U.S. Pat. No. 5,654,043 to Shao et al. also concerns a system for treating a workpiece surface by causing ions to impact the workpiece surface. A gas is injected into an implantation chamber so that an ionizable gas occupies a region in close proximity to the workpiece surface. A plasma of implantation material is created within the interior region of the implantation chamber by repeatedly relatively biasing conductive electrodes with a sequence of pulses that both ionize the gas molecules injected into the chamber and accelerate the ionized gas molecules toward the implantation surfaces of the one or more workpieces. The disclosure of the '043 patent to Shao et al. is incorporated herein by reference.
A paper entitled "Dosimetry Measurement in Ion Implanters" copyright 1981 by Douglas M. Jamba discloses the use of a dosimetry cup to collect charges impacting a target during ion beam implantation. To more accurately measure the charge actually impacting the target, a biasing electrode located between a beam defining aperture and the entrance to the cup is used to suppress electrons and negative ions that are trying to leave the cup. The biasing electrode also suppresses electrons generated on the defining aperture that are trying to enter the cup, and repels electrons that are carried with the ion beam.
A paper entitled "High Current Dosimetry Techniques" copyright 1979 by C. M. McKenna discloses use of one or more dosimetry cups located adjacent to the target for in-situ measurements of the charge bombarding the target during ion beam implantation. Measurements made by such dosimetry cups are used to control the ion beam being used for implantation. McKenna further discloses a negatively biased cup collector surface used in conjunction with less negatively biased cup walls to repel unwanted electrons from entering the cup. However, McKenna's experiments showed that this type of electrical biasing caused eruptions on the silicon target due to electrical discharges aggravated by secondary electrons trapped on the magnetic field lines within the dosimetry cup.